Optical fibers are widely used in the field of telecommunications for transmitting signals. They essentially comprise an inner cylindrical region, called a core, within which a signal is transmitted, and an outer region surrounding the core, called cladding. The refractive index of the cladding region is lower than that of the core region, so as to confine the transmitted signal within the core region.
Typically, both the core and the cladding are made from silica glass material. The difference in refractive index between the core and the cladding is obtained by incorporating suitable additives, so called dopants, into the glass matrix of the core and/or cladding.
Typical examples of dopants used for modifying the refractive index of silica are germanium, aluminum and phosphorous, which increase its refractive index, and fluorine or boron, which decreases its refractive index.
If desired, the core of optical fibers can be further doped with particular substances capable of giving effects of optical amplification, such as rare earth ions. Rare earth ions have spectroscopic properties that are particularly suitable for the purpose. Among rare earths, erbium is the most frequently used component since its fluorescence spectrum has a band ranging between 1420 and 1650 nm, which corresponds to the third transmission window, centered at about 1550 nm, of a telecommunication signal.
As of late, there has been an increased interest in hydrogen induced losses in optical fibers. This is attributed to hydrogen reactions occurring at germaniun-related defect sites created during the addition of germanium as a dopant; A. Tomita & P. J. Lemaire, Electronics Letters, 17th Jan. 1985, Vol. 21, No. 2, pp. 71-72. Lemaire et al. disclosed in OFC/IOOC '93 Technical Digest TuL3 that such hydrogen induced losses do not usually constitute a problem for single mode fibers, however, they are of potential concern for highly doped fibers used in erbium doped fiber amplifiers (EDFA). AT&T Bell Laboratories first discovered that erbium doped fiber is susceptible to long-term degradation caused by hydrogen induced loss increases in installed optical fibers. In 1993, Lemaire et al. (OFC/IOOC '93 Technical Digest TuL3) confirmed that typical erbium doped fiber compositions were highly reactive when exposed to even low levels of ambient hydrogen. Erbium-doped fibers made by different manufacturers using different processing techniques showed that this high reactivity is inherent in the most widely used erbium-doped fiber compositions based on GeO2—Al2O3 co-doped host. This potential reliability problem is recognized and addressed in Telcordia requirements. For example, Telcordia specification (Bellcore GR-1312-Core, Issue 3, April 1999) per Section 8.1.3 of GR-1312-core requires the demonstration of 20 years of product life at 0.01 atm of hydrogen at 38° C. Therefore, reducing hydrogen aging is important for erbium-doped fibers and their use in EDFAs.
Rare earth ions, such as erbium, are of special interest because they can provide gain in the low loss window of long haul transmission fiber. Due to the nature of the erbium atom, the gain provided in this window is not flat, rather, it has a particular gain shape which is undesirable. In order to achieve gain flatness, gain-flattening filters are used successfully. One environmental concern for amplifiers that use erbium-doped fiber is exposure to hydrogen. Hydrogen can diffuse into the fiber core region where it can react with germanium and silicon defect centers to form OH groups, which cause optical loss in the wavelength region of interest. For erbium-doped fiber, this effect can cause the gain shape of the fiber to change and render the gain flattening filter useless for the application.
The state of the art discloses several approaches to reduce hydrogen aging problems. Hermetic fiber coating and deuterium passivation are two of the most widely accepted methods for reducing such hydrogen induced losses. However, these methods of reducing hydrogen-induced loss require further process time and costs.
It is an object of this invention to provide an optical fiber that has an improved resistance to hydrogen-induced optical loss.
Another object of this invention is to provide an optical fiber that has improved resistance to hydrogen-induced loss without prior passivation.
It is a further object of the invention to provide an erbium-doped fiber with improved gain and improved resistance to hydrogen-induced optical loss.